The Supreme Court on Monday debated what one liberal justice said would be a “radical” restructuring of organized labor by prohibiting states from requiring public employees to pay fees to the unions that represent them.

The case from Illinois concerns home-care workers and whether those who do not join the public employees union must pay compulsory fees to cover the cost of collective bargaining. The Supreme Court since 1977 has said states have the power to require such payments — about half of them use it — so long as the fees are not used for political purposes.

But William Messenger, an attorney for the National Right to Work Legal Defense Foundation, said the court should reconsider that precedent. Forcing public employees to support a union with which they might disagree violates their constitutional rights of association and free speech, he said.

“Our position is that in the public sector when government is involved, compulsory fees are illegal under the First Amendment,” Messenger said.

The case pits right-to-work supporters against labor unions and the Obama administration, and just as predictably mostly split the justices along ideological lines.

Less predictably, the justice who seemed to hold the balance on the question was Antonin Scalia. He was part of the conservative majority in a case two terms ago that seemed to invite a challenge to the court’s 1977 ruling in Abood v. Detroit Board of Education.

But in Tuesday’s oral arguments, Scalia indicated that Messenger might be asking for too much.

“What our cases say is you . . . can be compelled not to be a free rider — to [have to] pay for those items of bargaining that benefit you as well as everybody else,” Scalia said. “You don’t have to pay for stuff that is not within that description, stuff that doesn’t benefit you at all.”

The political significance of the case is clear and even acknowledged by the justices. Public employee unions have become a major player in Democratic politics, and disputes between the unions and Republican governors have become frequent and bitter.

Justice Elena Kagan mentioned the public controversy in Wisconsin that led to an unsuccessful attempt to recall Gov. Scott Walker, a Republican. And Justice Samuel Alito said the case from Illinois was prompted by a decision by disgraced former governor Rod Blagojevich, a Democrat, to reward the Service Employees International Union by declaring the workers state employees for collective-bargaining purposes.

“I thought the situation was that Governor Blagojevich got a huge campaign contribution from the union and virtually as soon as he got into office he took out his pen and signed an executive order that had the effect of putting, what was it, $3.6 million into the union coffers?” Alito asked Solicitor General Donald Verrilli.

Verrilli, representing the Obama administration, replied that the Illinois legislature later passed legislation to the same effect with bipartisan support.

Messenger told the court that the Illinois case presented serious problems because the workers, whose salary and benefits are set by the state but who are employed by the clients they serve, are part of a Medicaid program. That means, he said, that the union’s negotiations are really petitioning the government to change the Medicaid program and compel some workers to pay for speech with which they disagree.

But the liberal justices said the court’s precedents recognize a difference when the government is acting as an employer rather than as an authority that citizens can petition.

“What strikes me is that this is — I’m just going to use the word here — it is a radical argument,” Kagan told Messenger. “It would radically restructure the way workplaces across this country are run.”

Justice Stephen Breyer said the only reason the court would be able to find for overruling Abood would be to decide that government employees had no right to unionize or could not bargain over wages and benefits. He said Messenger was asking the court to “fashion, using the First Amendment as their weapon, a new special labor law for government employees.”

Conservative justices were equally tough on Washington lawyer Paul Smith, who argued on behalf of the union.

Alito and Justice Anthony Kennedy pointed out that government employees are different. The union’s views on how government should come up with more money to pay employees could clash with some workers’ political views, Kennedy said.

“I’m talking about whether or not a union can take money from an employee who objects to the union’s position on fundamental political grounds,” Kennedy said. “Government is getting bigger and bigger, and this is becoming more and more of an important issue to more people.”

Smith replied that the difference is that the non-members are being required to pay only for collective bargaining that benefits all the workers.

Alito wondered about a young person who was not concerned about pensions, but the unions making that a priority.

“So that employee who’s not a member of the union has to pay for the union to bargain with the state to achieve something that’s contrary to that person’s interest,” Alito said. “But you say that person is a free rider.”

Chief Justice John Roberts seemed to think that because the workers are paid with federal and state Medicaid money, that might be considered lobbying on a political issue.

While the justices spent almost all of the argument on the broader questions of public employee collective bargaining, the court has narrower ways to decide the case. The nonunion members also contend that they are not lawfully classified as state employees, and should not be part of the collective-bargaining unit.